In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an "optical waveguide." The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect. The most frequently used material for such a waveguide device is glass, which is formed into a fiber of specified dimensions.
As the development of optical circuits proceeded, it became necessary to have structures which could couple, divide, switch and modulate the optical waves from one waveguide device to another. A device of particular interest is the "Y-coupler", which is a "y" shaped device that couples signals together or divides them apart. One method used to form such a coupler is to fuse or melt two fibers together so that light from one fiber can pass to the connected fibers. However, in such a fusion process it is difficult to control the extent of fusion and the exact geometry and reproduciability of the final structure.
Another method used to form an optical coupling device involves the application of standard photolithographic processes and diffusion. By this prior art process, standard lithographic processes are used to define a pattern in a photoresist layer deposited on a chosen substrate. Then, an etchant is applied to etch the photoresist-defined pattern into the substrate. Next, a metal is deposited in the etched region by vacuum deposition. The photoresist pattern is then lifted off with an appropriate solvent, carrying with it unwanted metal deposits. The structure is then heated to diffuse the metal deposited in the etched region into the substrate, to form a waveguiding layer therein. In addition to the fact that many steps are involved in such a process, there is also a limitation on the thickness of the metal which may be deposited. First, since vacuum deposition is a relatively slow process, there is the limitation of the excessive amount of time required to deposit a thick layer of metal. Secondly, as more and more metal is depositied, new centers for deposition are created, resulting in an uneven deposit.
Another approach to coupling and branching has been taken by K. Kobayashi, R. Ishikawa, K. Minemura, and S. Sugimoto as reported in a publication entitled "Micro-Optics Devices for Branching, Coupling, Multiplexing, and Demultiplexing," in the Technical Digest of the 1977 International Conference on Integrated Optics and Optical Fiber Communication, July 18-20, 1977, Tokoyo, Japan. Kobayashi et al use light focusing rod lenses, or light guides with a parabolic refractive index distribution, to obtain branching, coupling, multiplexing, and demultiplexing. Such lenses must, however, be cut and polished to specification, which are costly processes. In addition, these lenses are designed for and function only at a specific wavelength and thus have limited applicability.
Still another approach to branching structures has been reported by F. Auracher, H. Boroffka, and R. Th. Kersten in an article entitled "Planar Branching Networks for Multimode and Monomode Glass Fiber Systems", Integrated Optics, a digest of technical papers presented at the topical meeting on Integrated Optics, Jan. 12-14, 1976, Salt Lake City, Utah. Auracher et al coated a quartz substrate with thin sheets of a commercially available light-sensitive material (specifically, Dupont's "Riston" /2/) and laminated these sheets to form a layer approximately 100 micrometers thick. Then, by standard photolithographic processes, a pattern was developed in the light-sensitive material. However, as Auracher et al discuss, the performance of the branching structure so formed was poor. Optical losses as well as the aging resistance of the exposed material were unsatisfactory.